Despite the successful administration of targeted therapy drugs, approximately 50% of HER2+ breast cancer patients fail to benefit from this therapy, and the reason for this poor response was a significant dysregulated expression of miRNAs [miR-23b-3p, miR-195-5p, miR-656-5p, and miR-340-5p] that led to trastuzumab resistance [155]. This evidence concerns the gene ERBB2 and breast cancer.